Sexually receptive female rats adopt a lordotic posture when mounted by males attempting intromission. The facilitating effects of ovarian steroids on lordosis response have been extensively studied. The role of classical great deal of investigative attention. More recently, a number of neuropeptides have been reported to affect lordosis response as well. We have discovered that the neuropeptides, oxytocin (OXY) and argininevasopressin (AVP), both potently facilitate sexual receptivity when administered intracerebroventricularly (ICV). These effects are estrogen dependent and dose related. We propose a number of investigations of the mechanisms and physiological relevance of nonapeptide-induced lordosis behavior. We will determine whether the nonapeptides act solely on the CNS or is enhanced by the nonapeptides alone or by fragments of these peptides produced by proteolysis. The behavioral potency of various analogs of OXY and AVP will be measured to determined whether brain receptors mediating the lordosis effects of nonapeptides are comparable to receptors mediating the lordosis effects in peripheral tissues. Brain sites at which OXY and AVP have competitive inhibitor analogs, we will determine whether OXY or AVP endogenous to the brain mediate the effects of estrogen and progesterone on sexual receptivity. We will assess the effects of various ovarian steroid conditions that affect lordosis behavior as well as estrous cycling on release, regional content and binding of OXY and AVP within the CNS as well as the effects of these conditions on the morphology of OXY or AVD containing neurons and the pattern of estradiol and progesterone binding on OXY and AVP neurons in the brain.